With the increase in communication demand, an optical network using a wavelength multiplexing technology (WDM: Wavelength Division Multiplexing) are widely used. The wavelength multiplexing technology is a technology which multiplexes and transmits a plurality of optical signals having different wavelengths. According to wavelength multiplexing technology, it is possible to multiplex 40 optical signals each having transmission speed of 40 (Gbps), and to transmit the multiplexed signal as a wavelength multiplexing optical signal of 1.6 (Tbps), for example.
As a transmission device using the WDM technology, an optical branch insertion device called ROADM (Reconfigurable Optical Add-Drop Multiplexer) is known. The optical branch insertion device has optical amplifiers in the input side and the output side of the wavelength multiplexing optical signal. The optical amplifier of the input side is called a pre-amplifier, and the optical amplifier of the output side is called a post-amplifier.
The optical amplifier determines a target ratio of an input level and an output level, i.e., gain, from a difference of the output level and input level of a desired light, and provide a control so that the gain becomes constant. For example, Japanese Laid-open Patent Publication No. 2004-23437 (hereinafter referred to as “Patent Document 1”) discloses a technology which outputs an ASE (Amplified Spontaneous Emission) light (spontaneous emission light) from the post-amplifier of a certain node to the pre-amplifier of an adjacent node, and sets the gain of the pre-amplifier based on a level of the received ASE light.
Japanese Laid-open Patent Publication No. 8-248455 (hereinafter referred to as “Patent Document 2”) discloses an optical amplifier in which optical amplification units which include an AGC (Automatic Gain Control) function are provided before and after a variable optical attenuator, respectively. Each of the optical amplification units includes a rare-earth doped fiber, an excitation light source which excites the rare-earth doped fiber, and a photodiode for monitoring a level of the light of the input side and the output side of the rare-earth doped fiber.